Add SCTP transport to the public API.

This involves inserting an extra layer between jsep_transport_controller
and the cricket::SctpTransportInternal layer. The objects at this layer
are reference counted.

Bug: chromium:818643
Change-Id: Ibed57c4a538de981cee63e0f7f1f319f029cab39
Reviewed-on: https://webrtc-review.googlesource.com/c/123884
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Steve Anton <steveanton@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26889}
diff --git a/api/peer_connection_interface.cc b/api/peer_connection_interface.cc
index 944b28e..244ee73 100644
--- a/api/peer_connection_interface.cc
+++ b/api/peer_connection_interface.cc
@@ -10,6 +10,7 @@
 
 #include "api/peer_connection_interface.h"
 #include "api/dtls_transport_interface.h"
+#include "api/sctp_transport_interface.h"
 
 namespace webrtc {
 
@@ -187,6 +188,12 @@
   return nullptr;
 }
 
+rtc::scoped_refptr<SctpTransportInterface>
+PeerConnectionInterface::GetSctpTransport() const {
+  RTC_NOTREACHED();
+  return nullptr;
+}
+
 PeerConnectionInterface::BitrateParameters::BitrateParameters() = default;
 
 PeerConnectionInterface::BitrateParameters::~BitrateParameters() = default;